Customize Data-Access Layer Code Generation

Table of Contents

1. Introduction
2. What is a Data Access Layer?
3. Creating Custom Annotations
   • The @FileDBGenerated Annotation
   • The @Persistent Annotation
   • The @UniqueKey Annotation
4. Creating the Annotation Processor
   • Extracting Information from Annotated Classes
   • Creating Generic Classes
5. Conclusion

Introduction

In this article, we will explore the concept of a Data Access Layer (DAL) and learn how to create custom annotations to generate the data access layer. We will explain the purpose and functionality of each annotation and guide you through the process of creating the annotation processor. By the end of this article, you will have a clear understanding of how to use custom annotations to generate the data access layer.

What is a Data Access Layer?

The Data Access Layer (DAL) is the part of an application that directly interacts with the database. It provides methods for adding, updating, deleting, and retrieving data, commonly referred to as CRUD (Create, Read, Update, Delete) operations. The DAL acts as an intermediary between the application and the database, abstracting away the underlying database operations and providing a convenient interface for the application to work with data.

Creating Custom Annotations

The @FileDBGenerated Annotation

The @FileDBGenerated annotation will be used to mark model classes for which we want to generate a Data Object Model (DOM) at compilation time. This annotation will be placed on the model class and will signal to our processor that we need to generate a DOM for that class. The annotation will have a retention policy of source to ensure that it is discarded after compilation. Additionally, it will only be applicable to classes, so the target will be specified as ElementType.TYPE.

The @Persistent Annotation

The @Persistent annotation will indicate whether a field needs to be persisted in the database or not. This annotation will be used in combination with the @FileDBGenerated annotation. It will have a retention policy of source and will be applicable to fields only (specified as ElementType.FIELD). The annotation can optionally take a parameter named key, allowing the user to specify different names for the field in their code and the corresponding database column.

The @UniqueKey Annotation

The @UniqueKey annotation will specify whether a field is a unique key. This annotation will also be used in combination with the @FileDBGenerated annotation to identify fields that serve as unique keys. It will have a retention policy of source and will be applicable to fields only (specified as ElementType.FIELD).

Creating the Annotation Processor

To create the annotation processor, we will start by implementing the AbstractProcessor class and overriding its process method. This method will be invoked by Java with a set of annotations and the RoundEnvironment, which allows us to extract relevant information for further processing.

Within the processor, we will iterate through the annotated classes and extract the persisted and unique key fields within each class. We will also extract the name of the model class and its package name, which will be used to name the generated classes and determine their package.

Once the required information is gathered, we will create the concrete classes required for the data access layer. This includes an interface with thread operation methods, a DTO class containing the persisted and unique key fields, custom exception classes (RequestException and ServiceException), and the implementation class for the data access layer.

Conclusion

Creating custom annotations to generate the data access layer can greatly simplify the process of working with databases in your applications. By following this guide, you have learned how to create custom annotations, create the annotation processor, and generate the necessary classes for the data access layer. With this knowledge, you can enhance the efficiency and maintainability of your database operations.